1,721,033 research outputs found
Kinetic and mechanism study of the spontaneous, solvent- and base-catalyzed degradation of the precursor of the β-nitro alcohol metaraminol by combining HPLC/electronic circular dichroism/theoretical methods
No full textChiral beta-nitro alcohols are key intermediates in the synthesis of a wide range of active pharmaceutical ingredients. Despite their massive use for pharmaceutical applications, in-depth kinetics studies concerning their stability during formation and transformation reactions are scarce in the literature. In this study, the (1R,2S)-1-(m-benzyloxy)-2-nitro-1-propanol) (BNA), the precursor of the metaraminol, was selected as a molecular model and the retro-Henry reaction was explored by a multidisciplinary approach involving HPLC, electronic circular dichroism and theoretical methods.The enantio-, diastereo-, and chemo-selective high-performance liquid chromatographic method for deter-mining the purity of beta-nitro alcohol during its formation and degradation is based on the use of an amylose-derived chiral stationary phase under normal-phase eluent conditions. The influence of various factors (e.g. temperature, type of reaction solvent, basic and acid catalysts) on the degradation kinetics has been investigated. The retro-Henry reaction was found to be the major degradation of BNA, under spontaneous, solvent-and base-catalyzed conditions, resulting in the formation of its precursors 3-benzyloxybenzaldehyde and nitroethane
A chromatographic study on the exceptional chiral recognition of 2-(benzylsulfinyl)benzamide by animmobilized-type chiral stationary phase based on cellulose tris(3,5-dichlorophenylcarbamate)
No full textIn previous studies, Okamoto et al. described the results of the exceptionally large chiral recognitionof 2-(benzylsulfinyl)benzamide onto a coated-type chiral stationary phase based on cellulose tris(3,5-dichlorophenylcarbamate). As a continuation and deepening of those studies, here the sulfoxide and asmall set of its structural analogues were analyzed on the commercially available Chiralpak IC-3 chiralstationary phase based upon the same polysaccharide derivative as the chiral selector but immobilizedonto silica support.The chromatographic results obtained using different mobile phases consisting of pure methanol,ethanol and 2-propanol or binary mixtures n-hexane-2-propanol, which are prohibited with the pro-genitor coated-type chromatographic support, permitted to identify the NH2of the amide group as thekey structural element of the (S)-enantiomer of 2-(benzylsulfinyl)benzamide for obtaining a very highaffinity for the IC-3 chiral stationary phase
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
A New C2-Symmetric Atropisomeric Thiophene-Based Monomer for Inherently Chiral Electroactive Materials: Synthesis, HPLC Resolution, and Absolute Configuration Assignment
Full text linkHerein, we report on the synthesis and high-performance liquid chromatography (HPLC) resolution of a new atropisomeric C2-symmetry chiral monomer based on the 3,3′-bithiophene core, which was developed to produce novel, inherently oligomeric chiral electroactive materials. The analytical enantioseparation was optimized using the cellulose-type Chiralpak IB-3 column and a mixture of n-hexane–methanol–dichloromethane 90:5:5 (v/v/v) as the mobile phase. During the scale-up of the enantioseparation analytical conditions to a semipreparative level, remarkable deformations in the HPLC profile, such as peak splitting and plateau zones between enantiomeric peaks, were observed. We demonstrate the effects of sample diluent as they relate to distorted peak profiles, as well as provide experimental solutions to prevent the disturbing phenomenon. The optimized chromatographic conditions were exploited to collect milligram amounts of the enantiopure sample, which was submitted to chiroptical and stereochemical characterization studies
3-(Phenyl-4-oxy)-5-phenyl-4,5-dihydro-(1H)-pyrazole: a fascinating molecular framework to study the enantioseparation ability of the amylose (3,5-dimethylphenylcarbamate) chiral stationary phase. Part II. Solvophobic effects in enantiorecognition process
No full textThe enantiomers of five chiral compounds incorporating the 3-(phenyl-4-oxy)-5-phenyl-4,5-dihydro-(1H)-pyrazole scaffold and differing only in the linear alkyl chain (varying in length from 1 to 5 carbon atoms) linked to the oxygen atom were directly separated on the amylose (3,5-dimethylphenylcarbamate) based Chiralpak AD-3 chiral stationary phase. The effects of the mobile phase composition, the structure of the analytes and temperature on the retention and enantioselectivity were investigated. It was found that the enantiomeric separations were in all cases enthalpy-driven and disfavored by entropic term. U-shape curves obtained by plotting the chromatographic data versus the alcoholic percentage in n-pentane-methanol and n-hexane-ethanol mobile phases highlighted that, at higher alcohol concentrations, solvophobic interactions were operative in the retention mechanism. The unusual trend of such curves was linked to the nature of alkyl chain of the pyrazolines and it was indicative of the solvophobic contribution to the achievement of a high degree of enantioseparation
Kinetic Study on the Base-Catalyzed Imine-Enamine Tautomerism of a Chiral Biologically Active Isoxazoline Derivative by HPLC on Amylose Tris(3,5-dimethylphenylcarbamate) Chiral Stationary Phase
No full textIsoxazoline is a nitrogen- and oxygen-containing five-membered heterocyclic scaffold with diverse biological profiles such as antimicrobial, fungicidal, anticancer, antiviral, analgesic and anti-inflammatory activity. Accordingly, the use of this peculiar structural framework in drug discovery is a successful strategy for the development of new drug candidates. Here, a chiral saccharin/isoxazoline hybrid was considered to investigate the tendency of the imine moiety of the heterocyclic ring to tautomerize to the enamine form in the presence of a basic catalyst. The pseudo-first-order rate constants for the base-catalyzed tautomerization process were measured in different solvents and at different temperatures by off-column kinetic experiments based on the amylose (3,5-dimethylphenylcarbamate)-type chiral stationary phase. The kinetic results obtained in this study may be a useful aid in the perspective of designing experimental conditions to control the stereointegrity of these types of pharmacologically active compounds and drive their synthesis toward the preferred, imine or enamine, tautomer
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
3-(Phenyl-4-oxy)-5-phenyl-4,5-dihydro-(1H)-pyrazole: a fascinating molecular framework to study the enantioseparation ability of the amylose (3,5-dimethylphenylcarbamate) chiral stationary phase. Part I. Structure-enantioselectivity relationships
Full text linkChiral stationary phases (CSPs) based on amylose (3,5-dimethylphenylcarbamate) (ADMPC) exhibit awide-range of enantioselectivity in high-performance liquid chromatography (HPLC) and supercriticalfluid chromatography (SFC). Although this class of CSPs has been extensively used, chiral discriminationsat receptorial level, which are useful to develop predictive molecular models, have been rarely reportedin the literature.Herein, we describe the results obtained in the enantioselective HPLC of a set of six C5-chiral 4,5-dihydro-(1H)-pyrazole derivatives on the ADMPC-based Chiralpak AD-3 CSP (CSP) under normal-phaseand polar organic conditions. Using pure methanol as a mobile phase the exceptional enantioseparationfactor value of 50 at 25◦C was found for one of the investigated analytes. To the best of our knowledge, theenantiomeric bias represents the most outstanding enantioseparation ever recorded on ADMPC-basedCSPs.Systematic variations in chemical groups in specific positions of the 3-(phenyl-4-oxy)-5-phenyl-4,5-dihydro-(1H)-pyrazole molecular framework resulted in peculiar changes in retention andenantioselectivity. A careful analysis of the chromatographic data permitted to advance some hypothesesconcerning the role played by the individual chemical groups in determining the exceptional enantiosep-aration.In particular, under methanol-rich mode, the prenyl moiety of the second eluted enantiomer of thebetter resolved analyte was recognized as a critical structural element to establish direct and favorablesolvophobic interactions with apolar portions of selector
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